The peroxidase and autoradiographic methods for tracing neural connections will be used to study the organization of connections between the auditory cortex, the thalamus and the inferior colliculus in the cat. Injections of tracer substances will be made through a recording micropette to identify the injection site on physiological grounds. The recording pipette will be used to monitor the responses of neurons to auditory stimuli, so that the pattern of connections can be related to the tonotopic maps in associated structures. The layers in the auditory cortex (AI) that give rise to efferent connections will be demonstrated, as well as the cell types that contribute to these projections. The Golgi method will be used to study the neuronal organization of the individual layers in the auditory cortex, and the results of these observations will be correlated with the known patterns of projection from each layer. The layers that receive thalamic and callosal input will also be demonstrated using the autoradiographic method. Finally, the method of transneuronal transport will be used to study the connections of the eighth nerve after intra-cochlear injection of tritiated proline.